I installed a Mustang A9L computer in my 1988 F-150 during my mass-air conversion. Now the oil pressure gauge shows all the way to the right (was working fine before). I assume the Mustang sending unit is different from the F-150 and I need to solder a resistor in line with the lead. Does anyone know what size the resistor needs to be (if I'm correct)?

 

Good job on the conversion.
Looks like you may have the oil pressure sender wire crossed somewhere in the wire loom.
The oil press sender in the Mustang and the F150 operate the same. It is just an on and off switch.

You can unplug the sender and see if the guage still pegs out. If not, then it's the sender.

 

I think in the 88s the sending unit is still a true pressure gauge, not just a switch (at least it did vary with rpms with the original computer). Autozone also lists the two sending units as different part numbers. I'll unplug the wire to see what happens to the gauge (doh, should've done that right away)! Thanks!

 

The oil press. sender is not an input to the PCM.
You could always run a manual guage.

 

I looked at the EVTM for the '88 F150 and the sender (oil press) is variable resistance. I could not find the EVTM for the Mustang.
I would assume it's the same.

 

That's what I thought, too. And none of the people that have done the conversion before me, have mentioned anything similar. This weekend I'll go and check a few things out. Maybe I did wire something wrong

 

I checked out the connection. If I remove the wire from the oil pressure sending unit, the gauge reads 'low', if I plug it back in, it reads way over 'high' (both times with the engine running). It seems as if there needs to be a resistor in-line. Anybody have any rough ballpark idea as to what size the resistor needs to be?